Electrically conductive paste for front electrode of solar cell and preparation method thereof

ABSTRACT

The present invention provides an electrically conductive paste for a front electrode of a solar cell and a preparation method thereof. The electrically conductive paste is composed of a glass-free corrosion binder, a metallic powder and an organic carrier. The corrosion binder is one or more Pb—Te based crystalline compounds having a fixed melting temperature in a range of 440° C. to 760° C. During a sintering process of the electrically conductive paste for forming an electrode, the glass-free corrosion binder is converted into a liquid for easily corroding and penetrating an antireflective insulating layer on a front side of the solar cell, so that a good ohmic contact is formed. At the same time, the electrically conductive metallic powder is wetted, and the combination of the metallic powder is promoted. As a result, a high-conductivity front electrode of a crystalline silicon solar cell is formed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/894,908 filed on Jun. 21, 2013 and claims priority to a PCTApplication No. PCT/CN2013/073098, filed on Mar. 22, 2013, commonlyassigned and incorporated as reference for all purposes.

The subject matter of the present application is related to China PatentApplication No. 201210360864.5, filed on Sep. 25, 2012, by Ran Guo, U.S.patent application Ser. No. 13/730,939, filed on Dec. 28, 2012, by RanGuo, and U.S. patent application Ser. No. 13/787,997, filed on Mar. 7,2013, by Xiaoli Liu et al., commonly assigned and incorporated byreference herein to their entireties for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to the technical field of solar cells, andmore particularly to an electrically conductive paste for a frontelectrode of a solar cell and a preparation method thereof.

Solar energy is an inexhaustible source of clean energy. With theincreasing depletion of coal, oil and other non-renewable energy,development and use of solar energy have become a big trend. Use ofsolar cells is a typical means of using solar energy, and crystallinesilicon solar cells for which industrial production has been achievedare one type of solar cells.

As the most important core part of crystalline silicon solar cells, acell sheet needs to collect and export a current generated under lightirradiation, so two electrodes need to be fabricated on a front side anda back side of the cell sheet. Many methods can be used to fabricate theelectrodes, among which screen printing and co-sintering are the mostcommonly used production processes currently.

In a crystalline silicon solar cell, an electrically conductive pastefor a front electrode, an electrically conductive paste for a backelectrode, and a paste for an aluminum back-surface field are coated ona silicon chip by adopting screen printing, and a front electrode isformed on the front side of the silicon chip through co-sintering.

The co-sintered electrode of a crystalline silicon solar cell isrequired to have strong adhesion, have no ash falling and no deformationof the silicon chip, and be easy to weld and convenient to collect andexport the current generated under light irradiation by means of a wire.Compared with the electrically conductive paste for a back electrode ofthe silicon solar cell, the electrically conductive paste for a frontelectrode of a silicon solar cell is required to have the ability topenetrate the silicon nitride antireflective layer.

In the prior art, the electrically conductive paste for the frontelectrode of a crystalline silicon solar cell is composed of a silverpowder, a glass frit, an additive and an organic carrier. The glassfrit, as an inorganic adhesive, binds the high-conductivity silverpowder and the silicon substrate together, and during co-sintering, themolten glass frit etches and penetrates the silicon nitrideantireflective layer, so that a good contact is formed between thesilver powder and the silicon substrate. Generally, the glass frit inthe paste has the following effects: (1) wetting the metallic powder topromote the sintering of the metallic powder; and (2) etching theantireflective layer to promote the contact of the metal and the siliconsurface and ensure the binding effect between the metal and the siliconsurface. In order to achieve a good ohmic contact of the metallic powderand the silicon surface, the antireflective layer is required to beetched through but not penetrate into a P-N junction region. In theselection of the glass frit, the composition, softening point, thermalexpansion coefficient, wetting properties and amount will affect thephysical and chemical changes in the sintering process, therebyaffecting the performance of the solar cell. In the sintering process,the glass frit is gradually softened, and within a short process cycle,usually 1 to 2 minutes, part of the softened glass frit remains aroundthe metallic powder and flows, and the other part of the softened glassfrit flows to reach the antireflective layer at the bottom and induces areaction. If the content of the glass frit is low, full contact andcomplete reaction of the glass frit and the antireflective layer cannotbe ensured. If it is ensured that the antireflective layer is completelypenetrated, a sufficient amount of the glass frit needs to be added. Thehigher the amount of the glass frit is, the lower the relative contentof the electrically conductive metallic phase is, and the lower theprobability of contact of metallic particles is, resulting in seriousdeterioration of conductivity. If a glass frit with a low softeningpoint such as a softening point of lower than 400° C. is selected toensure that a sufficient amount of glass frit is deposited on thesurface of the antireflective layer in the entire process, and reactwith the antireflective layer completely. But excessively-earlysoftening of the glass frit can clog the communicating pores in themetallic powder, thereby hindering the effective discharge of theorganic carrier.

Presently, a Pb—Si based glass material is widely used as the glass fritin the front electrode paste. At the same time, Pb oxide, Te oxide andother oxides or fluorides are used to go through a series of processesof melting, mixing and quenching, to prepare a Pb—Te—O glass material.However, regardless the use of various glass frit materials, due torestrictions of the physical properties of the glass frit, the abovetechnical problems still exist, resulting in process difficulties withnarrow windows in preparation of a suitable glass frit and a subsequentconductive paste. Therefore, improved techniques are desired for themanufacture of an electrically conductive paste for forming frontelectrodes of semiconductor devices.

BRIEF SUMMARY OF THE INVENTION

The objective of the present invention is to improve an electricallyconductive paste for the manufacture of an electrode on a semiconductorsurface. The electrically conductive paste is characterized by a strongadhesion property by adding a full-crystal-based corrosion binder freeof any glass frit for facilitating a formation of an electrode withexcellent metal-semiconductor electrical contact. In particular, theelectrically conductive paste can be applied for forming a frontelectrode of a silicon-based solar cell with overall enhanced lightconversion efficiency. An alternative objective of the present inventionis to provide a method of making the electrically conductive pastefree-from any glass frit in additives using a simple process witheasy-controlled conditions and reduced production cost.

In a specific embodiment, the present invention provides an electricallyconductive paste for a front electrode of a solar cell. The electricallyconductive paste includes a metallic powder having a weight ratio of 70wt % to 95 wt % based on a given total weight. Additionally, theelectrically conductive paste includes a glass-free corrosion binderhaving a weight ratio of 0.5 wt % to 12 wt % based on the given totalweight. Moreover, the electrically conductive paste includes an organiccarrier having a weight ratio of 5 wt % to 25 wt % based on the giventotal weight. The glass-free corrosion binder is a Pb—Te—O crystalcompound having a fixed melting temperature between 440° C. and 760° C.The metallic powder and the glass-free corrosion binder are randomlydispersed in the organic carrier.

In another specific embodiment, the present invention provides a methodfor forming a conductive paste. The method includes providing aplurality of metal particles with a weight composition ranging from 70wt % to 95 wt % based on a predetermined total weight. The methodfurther includes providing an organic carrier with a weight compositionranging from 5 wt % to 25 wt % based on the predetermined total weight.Additionally, the method includes providing a glass-free corrosionbinder made from a plurality of Pb—Te—O-based crystalline particles witha weight composition ranging from 0.5 to 12 wt % based on thepredetermined total weight. The method further includes mixing theplurality of metal particles, the glass-free corrosion binder, and theorganic carrier to form a mixture material. Furthermore, the methodincludes grinding the mixture materials to obtain a conductive paste.

In an alternative embodiment, the present invention provides methods formaking the electrically conductive paste, which is to mix the metallicpowder, glass-free corrosion binder, and organic carrier followed bygrinding so that the metallic powder and the glass-free corrosion binderare uniformly randomly dispersed in the organic carrier. The glass-freecorrosion binder of the present invention is a glass-free Pb—Te—O basedcrystalline compound, which is made by using one of the followingmethods: chemical reaction method, chemical vapor phase method,high-temperature melting reaction method, wet method and vacuum meltingmethod. The glass-free corrosion binder of the present inventionincludes one or a combination of two or more selected from the followingglass-free Pb—Te—O based crystalline compounds: PbTe₄O₉, PbTeO₃.0.33H₂O,PbTeO₃, PbTeO₄, PbTe₃O₇, PbTe₅O₁₁, Pb₂TeO₄, Pb₂Te₃O₇, Pb₂Te₃O₈, Pb₃TeO₅,Pb₃TeO₆, Pb₃Te₂O₈.H₂O, Pb₄Te_(1.5)O₇, Pb₅TeO₇, Pb₅TeO₇, Pb₆Te₅O₁₈.5H₂O,PbTe₂O₅, PbH₄TeO₆, PbTeCO₅ and Pb₃TeN₂O₈. The glass-free corrosionbinder is a power with particles at least in one shape selected fromsphere, droplet, aciculate, dendritic-shape, massive, spherical-shape,flake, granular-shape, and colloidal-particle-shape. The particles havesizes in a range of 0.1 to 5.0 μm in most applications, but has sizes ina range of 0.1 to 15.0 μm in some applications.

In another alternative embodiment, the present invention provides amethod for manufacturing a front electrode of a semiconductor device.The method includes providing a semiconductor device including aninsulation surface coating and printing an electrically conductive pasteoverlying a patterned contact region of the insulation surface coating.The electrically conductive paste includes a metallic powder with aweight composition ranging from 70 to 95 wt % based on a given totalweight of the electrically conductive paste, a glass-free corrosionbinder made from a plurality of Pb—Te—O-based crystalline particles witha weight composition ranging from 0.5 to 12 wt % based on the giventotal weight, and an organic carrier with a weight composition rangingfrom 4.5 to 25 wt % based on the given total weight. The glass-freecorrosion binder is one or a combination of two or more Pb—Te—O basedcrystalline compounds, having a fixed melting temperature in a range of440° C. to 760° C. The method further includes sintering theelectrically conductive paste overlying the patterned contact region ofthe insulation surface coating. The sintering process includes a step ofdrying the electrically conductive paste at a first temperature rangefrom 180° C. to 260° C. for 30 s up to 70 s. The sintering processfurther includes a step of heating up to a second temperature range from720° C. to 950° C. for 20 s up to 50 s and a step of cooling back to 25°C. to form an electrode. The drying and heating from the firsttemperature range to the second temperature range are associated withreleasing of the organic carrier, melting of the glass-free corrosionbinder at the fixed melting temperature after the releasing of theorganic carrier, and forming of a metallic bulk from the metallic powderwet by molten glass-free corrosion binder. The molten glass-freecorrosion binder induces etch-removing of the insulation surface coatingat the patterned contact region to form an ohmic contact between themetallic bulk and the crystalline silicon solar cell.

The electrically conductive paste used for the manufacture of front sideelectrodes on solar cell light receiving surface comprises full-crystalcorrosion binder and free from any glass material as a binding additivein the paste. By controlling proper metal-oxide materials with selectiveweight ratio among several ingredients and crystal particle sizes duringthe preparation of the full-crystal glass-free corrosion binder, andfurther by controlling the way of mixing with metallic powder andorganic carrier, the conductive paste bearing this full-crystalglass-free corrosion binder can be subjected to a broader range ofsintering process conditions to form electrodes on solar cells withgreatly reduced series resistance and enhanced photovoltaic conversionefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description given herein below for illustration only, and thusare not limitative of the present disclosure, and wherein:

FIG. 1 is a schematic view of a process of preparing a front electrodeby using an electrically conductive paste of the present invention.

FIG. 2 is a partial structural view of a crystalline silicon solar cellprinted with an electrically conductive paste before sintering.

FIG. 3 is a partial structural view of a crystalline silicon solar cellafter an electrically conductive paste is sintered.

FIG. 4 shows a cooling curve in preparing a Pb—Te—O crystallineglass-free corrosion binder.

FIG. 5 is an exemplary diagram of XRD measured from a Pb—Te—O-basedcrystalline compound glass-free corrosion binder, in which sharpdiffraction characteristic peaks exist in a small angle range.

FIG. 6 is an exemplary diagram of XRD measured from a Pb—Te—O-basedglass frit, in which a bump with a wide distribution and low intensityexists in a small angel range, and no sharp diffraction characteristicpeaks exist.

FIG. 7 is a schematic view of a process of preparing an electricallyconductive paste of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an electrically conductive paste for afront electrode of a solar cell and a preparation method thereof, and amethod for preparing a front electrode of a crystalline silicon solarcell by using the electrically conductive paste.

The electrically conductive paste for a front electrode of a solar cellaccording to the present invention includes the following components:

-   -   a metallic powder, having a weight ratio of 70 wt % to 95 wt %        in the electrically conductive paste for a front electrode of a        solar cell;    -   a glass-free corrosion binder, having a weight ratio of 0.5 wt %        to 12 wt % in the electrically conductive paste for a front        electrode of a solar cell; and    -   an organic carrier, having a weight ratio of 5 wt % to 25 wt %        in the electrically conductive paste for a front electrode of a        solar cell.

The glass-free corrosion binder is a Pb—Te—O based crystalline compoundhaving a melting temperature in a range of 440° C. to 760° C.; and themetallic powder and the glass-free corrosion binder are randomlydispersed in the organic carrier. The glass-free corrosion binder of thepresent invention includes one or a combination of two or more selectedfrom the following Pb—Te—O based crystalline compounds: PbTe₄O₉,PbTeO₃.0.33H₂O, PbTeO₃, PbTeO₄, PbTe₃O₇, PbTe₅O₁₁, Pb₂TeO₄, Pb₂Te₃O₇,Pb₂Te₃O₈, Pb₃TeO₅, Pb₃TeO₆, Pb₃Te₂O₈.H₂O, Pb₄Te_(1.5)O₇, Pb₅TeO₇,Pb₅TeO₇, Pb₆Te₅O₁₈.5H₂O, PbTe₂O₅, PbH₄TeO₆, PbTeCO₅ and Pb₃TeN₂O₈.

Furthermore, a method for preparing the glass-free corrosion binderincludes: mixing a hot telluric acid solution (tellurous acid solution,tellurate solution or tellurite solution) and a lead acetate solution,where the molar ratio of Te to Pb in the solution is in a range of0.1:10 to 10:0.1; stirring the mixed solution at a temperature in arange of 80° C. to 120° C. at a stirring speed in a range of 1,000 to1,500 r/min for reaction for 2 to 5 hours, to generate a precipitate,and collecting a solid through solid-liquid separation and washing, tillthe pH value of the filtrated water is in a range of 5 to 7, and dryingthe solid at 150° C. for 2 to 3 hours, to obtain the Pb—Te basedcrystalline compound, which is then pulverized and ground, to obtain aPb—Te based crystalline compound particle.

Furthermore, a method for preparing the glass-free corrosion binderincludes: continuously introducing Pb_(x)Te_(y) alloy vapor into areaction chamber filled with oxygen atmosphere; reacting at atemperature in a range of 1,000° C. to 1,400° C. for 1 to 4 hours, andnaturally cooling the resulting product to 25° C. to obtain the Pb—Te—Obased crystalline compound, which is then pulverized and ground, toobtain a Pb—Te—O based crystalline compound particle.

Furthermore, a method for preparing the glass-free corrosion binderincludes: in a non-reducing atmosphere (including oxygen atmosphere, airatmosphere, nitrogen atmosphere, and argon gas atmosphere), heating atellurium oxide and a lead oxide to a temperature in a range of 700° C.to 1,000° C., melting the tellurium oxide and the lead oxide forreaction, naturally cooling the resulting product to 25° C., and thenpulverizing and grinding, to obtain a Pb—Te—O based crystalline compoundparticle.

Furthermore, a method for preparing the glass-free corrosion binderincludes: in a vacuum atmosphere, melting a tellurium oxide and a leadoxide at a temperature in a range of 700° C. to 1,000° C., naturallycooling the resulting product to 25° C., and then pulverizing andgrinding, to obtain a Pb—Te—O based crystalline compound particle.

FIG. 1 is a schematic view of a process of preparing a front electrodeof a crystalline silicon solar cell by using an electrically conductivepaste of the present invention. The method for preparing a frontelectrode of a high-performance crystalline silicon solar cell accordingto the present invention includes providing a crystalline siliconsemiconductor device having an insulating film on an upper surface. Theinsulating film is an overlaying layer of one or a combination of two ormore selected from silicon nitride, titanium oxide, aluminum oxide andsilicon oxide. The method further includes printing an electricallyconductive paste for a front electrode of a solar cell on the insulatingfilm of the crystalline silicon semiconductor device. The electricallyconductive paste for a front electrode of a solar cell containsformulation components of the following parts by weight, based on thetotal weight of 100 parts: 1) a metallic powder 70 to 95 parts; 2) anorganic carrier 5 to 25 parts; and 3) a sum of a glass-free corrosionbinder and a glass frit 0.5 to 12 parts. The glass-free corrosion binderis one or a combination of two or more Pb—Te—O based crystallinecompounds, having a melting temperature in a range of 440° C. to 760° C.and a particle size in a range of 0.1 to 15.0 μm. Additionally, themethod includes sintering. The sintering process includes: first, dryingthe electrically conductive paste printed on the insulating film of thecrystalline silicon semiconductor device at a temperature in a range of180° C. to 260° C.; next, sintering by heating to 720° C. to 950° C.;and then cooling to obtain the electrically conductive electrode. In theprocess of sintering the electrically conductive paste for an electrode,the organic carrier is removed through evaporation and the glass-freecorrosion binder is converted into a liquid and easily flows, corrodes,and penetrates an antireflective insulating layer on a front side of acrystalline silicon solar cell, and wets the electrically conductivemetallic powder, thereby promoting the combination of the metallicpowder. As a result, a good ohmic contact is formed between theelectrically conductive metallic powder and the crystalline siliconsolar cell, and a high-conductivity front electrode of a crystallinesilicon solar cell is formed.

FIG. 2 is a partial structural view of a crystalline silicon solar cellprinted with an electrically conductive paste before sintering. Itshould be noted that FIG. 2 only shows an example, which should not beused to limit the scope of the present invention. As shown in FIG. 2,the crystalline silicon solar cell is composed of a semiconductorsubstrate 100, an emitter 102 and an insulating layer 110, a P-Njunction region exists between the semiconductor substrate 100 and theemitter 102, the electrically conductive paste 120 for an electrode isselectively printed on a partial surface of the insulating layer 110 byadopting screen printing. The electrically conductive paste for anelectrode includes a metallic powder 122, a glass-free corrosion binder126 and an organic carrier 128. According to the design and applicationof a solar cell, the printing width of the electrically conductive paste120 for an electrode may be in a range of 20 μm to 3 mm, and theprinting width of a thin electrode may be in a range of 20 to 70 μm, andthe printing width of the main electrode may be in a range of 1 to 3 mm.

FIG. 3 is a partial structural view of a crystalline silicon solar cellafter an electrically conductive paste is sintered. It should be notedthat FIG. 3 only shows an example, which should not be used to limit thescope of the present invention. As shown in FIG. 3, after sintering, theelectrically conductive paste is converted into an electrode 200. Thesintering process includes drying the electrically conductive paste byheating from 25° C. to a temperature in a range of 180° C. to 260° C.,and then heating to a temperature in a range of 720° C. to 950° C. forsintering, and cooling, to form the electrode 200. In the sinteringprocess, with the raise of the temperature, the organic carrier in theelectrically conductive paste is removed, and the glass-free corrosionbinder is melted into a liquid 210 and flows towards a surface of theinsulating layer 110, etches and penetrates the insulating film 110 on asurface of the crystalline silicon semiconductor device, so that themetallic powder directly contacts the substrate of the crystallinesilicon semiconductor device, and an ohmic layer 240 is formed. Themolten glass-free corrosion binder promotes the combination of themetallic powder, and a high-conductivity metallic powder combination 220is formed, which has a good ohmic contact with the semiconductor 102through the ohmic layer 240. As a result, a high-conductivity frontelectrode 200 of a crystalline silicon solar cell is formed. In anembodiment, the front electrode of the present invention includes ametallic powder uncoated with silver, which includes one or acombination of two or more selected from silver, gold, platinum, copper,iron, nickel, zinc, titanium, cobalt, chromium, aluminum, manganese,palladium and rhodium. In another embodiment, the front electrode of thepresent invention includes one or a combination of two or more selectedfrom copper, iron, nickel, aluminum, zinc, titanium, cobalt, chromiumand manganese coated with silver. In another embodiment, the frontelectrode of the present invention includes a mixture of a metallicpowder uncoated with silver and a metallic powder coated with silver,and the weight ratio of the metallic powder uncoated with silver to themetallic powder coated with silver is 5:95 to 95:5.

The components, the preparation method and the use of the electricallyconductive paste for a front electrode of a crystalline silicon solarcell according to the present invention are described in detail below.

I. Glass-Free Corrosion Binder

Conventional electrically conductive paste for a front electrode of acrystalline silicon solar cell composes a silver powder, a glass frit,an additive and an organic carrier. The electrically conductive paste isgenerally printed on a front side or a light irradiation side of thecrystalline silicon solar cell, and then is sintered to form a frontelectrode. In the sintering process, the glass frit in the electricallyconductive paste etches and penetrates an antireflective insulatinglayer on the front side or the light irradiation side of the crystallinesilicon solar cell, so that the silver powder contacts the substrate ofthe crystalline silicon solar cell to form a front electrode, where theantireflective insulating layer is made of, for example, siliconnitride, titanium oxide, aluminum oxide and silicon oxide or siliconoxide/titanium oxide. The glass frit used in the conventionalelectrically conductive paste for a front electrode of a crystallinesilicon solar cell is an amorphous structure material. In a heatingprocess, the glass frit gets soft first, which is referred to as asoftening temperature. The different types of glass frits have differentsoftening temperature. Atoms in the glass frit are arranged disorderly,and X-ray diffraction (XRD) measurement shows that a bump with widedistribution and low intensity exists and no sharp diffractioncharacteristic peaks exist, which is different from the situation of acrystalline compound.

The glass frit is generally prepared by heating oxide material ormaterials to a melting state followed by quenching the molten. Forexample, US Patent No. US2011/0308595 discloses an electricallyconductive paste for a front electrode of a crystalline silicon solarcell, in which a glass frit is made from a lead tellurium oxide(Pb—Te—O) material, the method for preparing of the glass frit includes:mixing a lead oxide and a tellurium oxide, heating the mixture to amolten state in an air atmosphere or oxygen atmosphere; then, quenchingthe molten mixture, and grinding, to obtain the led tellurium oxide(Pb—Te—O)-based glass frit. In another example, PCT Patent No.WO2012/129554 discloses an electrically conductive paste for a frontelectrode of a crystalline silicon solar cell, in which a Pb—Te—O-basedglass frit is used. A method for preparing the glass frit is aconventional glass preparation method, and includes: mixing a lead oxideand a tellurium oxide, heating the mixture to a molten state; thenquenching the molten mixture, and grinding, to obtain the Pb—Te—O-basedglass frit. In another example, PCT Patent No. WO2012/129554 disclosesthat atoms in glass frit are arranged disorderly, and XRD measurementshows that a bump with wide distribution and low intensity exists and nosharp diffraction characteristic peaks exist, which is different fromthat for a crystalline compound. The softening temperature of the glassfrit per WO2012/129554 is in a range of 300° C. to 800° C. For anotherexample, US Patent No. US2011/0232747 discloses an electricallyconductive paste for a front electrode of a crystalline silicon solarcell, in which a Pb—Te—O-based glass frit is used, and the method forpreparing the glass frit includes: mixing TeO₂, PbO and Li₂CO₃, heatingthe mixture to 900° C. to melt and keeping at 900° C. for one hour, andthen quenching the molten mixture to obtain the Pb—Te—O-based glassfrit. For another example, US Patent No. US2011/0232746 discloses anelectrically conductive paste for a front electrode of a crystallinesilicon solar cell, in which a Pb—Te—B—O-based glass frit is used, and amethod for preparing the glass frit includes: heating a lead, telluriumand boron mixture to a temperature in a range of 800° C. to 1,200° C. tomelt, and quenching the molten mixture to obtain the Pb—Te—B—O-basedglass frit. The glass frit is an amorphous material and does not have amelting temperature.

The glass-free corrosion binder of the present invention is acrystalline compound, has a melting point, and is different from theglass frit. The glass-free corrosion binder of the present invention isone or a combination of two or more selected from the following Pb—Te—Obased crystalline compounds: PbTe₄O₉, PbTeO₃.0.33H₂O, PbTeO₃, PbTeO₄,PbTe₃O₇, PbTe₅O₁₁, Pb₂TeO₄, Pb₂Te₃O₇, Pb₂Te₃O₈, Pb₃TeO₅, Pb₃TeO₆,Pb₃Te₂O₈.H₂O, Pb₄Te_(1.5)O₇, Pb₅TeO₇, Pb₆Te₅O₁₈.5H₂O, PbTe₂O₅, PbH₄TeO₆,PbTeCO₅ and Pb₃TeN₂O₈, and these crystalline compounds have a meltingtemperature in a range of 440° C. to 760° C. The glass-free corrosionbinder is crystalline compounds, has a typical crystal characteristics,and when being heated to the melting temperature, the crystal begins tomelt into a liquid and has no softening temperature, and the meltingpoint is related to the material composition.

The crystal glass-free corrosion binder is distinctly different from theconventional glass frit in terms of its internal atomic structure. Eachparticle in the prepared glass-free corrosion binder is a crystallinecompound having ordered atomic structure while each glass frit particleis in an amorphous structure with a random atomic network. Significantdifference can be shown by X-ray diffraction (XRD) measurement. XRD scanof a glass-free corrosion binder sample yields several sharp peaks atcertain diffraction angles that are specifically associated with thecorresponding crystalline compound. While XRD scan of a glass fritmaterial shows a mostly flat distribution and only a small bump of lowstrength near the small diffraction angles region. Additionally, thestructural difference between the crystalline glass-free corrosionbinder and the glass frit can be directly revealed by transmissionelectron microscopy (TEM) image. When a TEM image of the crystallinecompound of any one of the glass-free corrosion binder added into theconductive paste of the present invention is taken, it demonstrates asurface with ordered atomic arrangement. But TEM image of any glass fritexhibits a surface with disordered atomic arrangement.

Additional difference between crystalline glass-free corrosion binderand glass frit may resulted from their preparation methods, even thoughthey may be started from substantially same oxide material with same orvery similar compositions. Conventional glass frit is typically formedby first heating the oxide materials till melt followed by quenching themelt in certain processes, which are different with the methods ofmaking the glass-free corrosion binders as described above.

The glass-free corrosion binder of the present invention is prepared byone of the following methods: liquid phase chemical reaction method,gas-phase chemical reaction method, melting controlled cooling methodand vacuum melting controlled cooling method. The glass-free corrosionbinder has a shape of one or a combination of two or more selected fromsphere, sphere-like shape, sheet, particle and colloidal particle. Thesize of the glass-free corrosion binder of the present invention is notparticularly limited, and in an embodiment, the size is less than 15 μm;in an embodiment, the size is less than 5 μm; in another embodiment, thesize is less than 3 μm; in another embodiment, the size is in a range of0.1 to 2.0 μm; in another embodiment, the size is in a range of 0.3 to1.7 μm.

In an exemplary embodiment, the glass-free corrosion binder of thepresent invention may be prepared by using a chemical reaction methodas: mixing a hot telluric acid solution (tellurous acid solution,tellurate solution or tellurite solution) and a lead acetate solution,where the molar ratio of Te to Pb in the solution is in a range of0.1:10 to 10:0.1; stirring the mixed solution at a temperature in arange of 80° C. to 120° C. at a stirring speed in a range of 1,000 to1,500 r/min for reaction for 2 to 5 hours, to generate a precipitate,and collecting a solid through solid-liquid separation and washing, tillthe pH value of the filtrated water is in a range of 5 to 7, and dryingthe solid at 150° C. for 2 to 3 hours, to obtain the Pb—Te basedcrystalline compound, which is then pulverized and ground, to obtain aPb—Te based crystalline compound particle. As well known to persons ofordinary skill in the art, by changing the chemical reaction conditionsin the exemplary embodiment, including changing the chemical componentsor the reaction temperature or time, Pb—Te based crystalline compoundswith similar properties can be obtained. In another exemplaryembodiment, the glass-free corrosion binder of the present invention maybe prepared by using a gas-phase chemical reaction method, and thepreparation process includes: continuously introducing Pb_(x)Te_(y)alloy vapor into a reaction chamber filled with oxygen atmosphere;reacting at a temperature in a range of 1,000° C. to 1,400° C. for 1 to4 hours, and naturally cooling the resulting product to 25° C. to obtainthe Pb—Te based crystalline compound, which is then pulverized andground, to obtain a Pb—Te based crystalline compound particle. As wellknown to persons of ordinary skill in the art, by changing the chemicalreaction conditions in the exemplary embodiment, Pb—Te based crystallinecompounds with similar properties can be obtained. In another exemplaryembodiment, the glass-free corrosion binder of the present invention maybe prepared by using a solid-phase reaction method, and the preparationprocess includes: in a non-reducing atmosphere (including oxygenatmosphere, air atmosphere, nitrogen atmosphere and argon gasatmosphere), heating a tellurium oxide and a lead oxide to a temperaturein a range of 700° C. to 1,000° C., melting the tellurium oxide and thelead oxide for reaction, naturally cooling the resulting product to 25°C., and then pulverizing and grinding, to obtain a Pb—Te basedcrystalline compound particle. As well known to persons of ordinaryskill in the art, by changing the chemical reaction conditions in theexemplary embodiment, Pb—Te based crystalline compounds with similarproperties can be obtained. For example, a tellurium oxide and a leadoxide may be melted at a temperature of 700° C. or less or a temperatureof 1,000° C. and more, a flowing protection gas (such as N₂, CO₂ and Ar)that is not heated flows through the surface of the molten to acceleratethe cooling rate; or a protection gas (such as N₂, CO₂ and Ar) that isheated flow through the surface of the molten to decrease the coolingrate, to obtain a Pb—Te—O-based crystalline compound. In anotherexemplary embodiment, the glass-free corrosion binder of the presentinvention may be prepared by using a vacuum melting controlled coolingmethod, and the preparation process includes: in a vacuum atmosphere,melting a tellurium oxide and a lead oxide at a temperature in a rangeof 700° C. to 1,000° C., naturally cooling the resulting product to 25°C., and then pulverizing and grinding, to obtain a Pb—Te basedcrystalline compound particle. As well known to persons of ordinaryskill in the art, by changing the chemical reaction conditions in theexemplary embodiment, Pb—Te—O based crystalline compounds with similarproperties can be obtained. For example, a tellurium oxide and a leadoxide may be melted at a temperature of 700° C. or less or a temperatureof 1,000° C. and more; a gas (such as N₂, CO₂ and Ar) may be used toflows through the surface of the molten to accelerate the cooling rate;or a hot gas (such as N₂, CO₂ and Ar) may be used to flow through thesurface of the molten to decrease the cooling rate, to obtain aPb—Te—O-based crystalline compound.

In one or more embodiments, the differences in structures andpreparation methods between a Pb—Te—O based glass material andcrystalline compounds are illustrated by following examples.

In an example according to an embodiment of the present invention,chemical compound TeO₂ and PbO in powder forms are mixed with a moleratio of 1:1. After mixing, the mixture is heated in air environment toabout 900° C. (which is above the melting point of either chemicalcompound) and is further held at the temperature for about 30 minutes toform a molten mixture. Then the molten mixture is removed from furnaceand cooled naturally in a room-temperature atmosphere to form a bulkmaterial. Upon the removal from the furnace, the temperature of themolten mixture is first cooled from 900° C. in the furnace to 732° C. inabout 3 seconds. FIG. 4 shows a plot of temperature drop after removalof the molten mixture from furnace versus its cooling time in a processfor preparing the glass-free corrosion binder according to an embodimentof the present invention. As shown, the recording starts from 732° C.and the temperature drops to 593° C. in the first 1 minute. The averagecooling rate is about 139° C./min. In a second minute, the temperaturedrops further to 504° C. with an average cooling rate of 89° C./min. Ina third minute, the temperature drops to 449° C. with an average coolingrate of 55° C./min. Furthermore, in a fourth minute, it drops to 416.8°C. with an average cooling rate of 32.8° C./min. At this stage, themolten mixture has become a bulk material. The bulk material is crushedinto small particles and further ball-milled into fine powders withsubstantially round shape having D₅₀ sizes ranging from 0.1 to 15 μm.Using XRD to exam samples of the fine powders, the resulted diffractionpattern (marked as M1) is shown in FIG. 5, plotted as the diffractionintensity versus 2θ (θ is X-ray incident angle) values across a rangefrom 10 degrees to 80 degrees. As shown, the plot yields many sharppeaks at certain 2θ values corresponding to characteristic peaksspecific for crystal compounds PbTeO₃, indicating that the glass-freecorrosion binder in powered form obtained via above preparation methodshows a substantially PbTeO₃ crystalline characteristic.

In another example, same chemical compounds TeO₂ and PbO in powders weremixed with the same mole ratio of 1:1. The powder mixture was placed ina crucible and heated in air atmosphere to form a melt, and maintainedat 900° C. for 30 min. Then the melt was cooled quickly by a quenchingmethod. In an implementation, the melt was quenched by pouring the meltdirectly on a stainless steel platen to obtain a bulk platelet accordingto methods presented in U.S. patent Ser. No. 13/100,550 and otherrelated references. In another implementation, the melt was quenched bypouring into deionized water to form a bulk material. The bulk plateletmaterial was crunched by grinding into small particles which are furtherball-milled into fine powders having D₅₀ sizes of 0.1˜15 microns. UsingXRD to exam samples of the fine powders, the resulted diffractionpattern (marked as M2) is shown in FIG. 6. As shown, the plotteddiffraction intensity versus 2θ (θ is X-ray incident angle) valuesacross a range from 10 degrees to 80 degrees yield a wide range of lowintensity curve with only a small bump near the small angular regions.This is a clear indication that no crystalline structure exists in thesefine powders, instead, the powders obtained by following conventionalpreparation method is predominantly glass material with an amorphousstructure.

In the above-mentioned two examples, although the use of the samecomposition and proportion of TeO₂ and PbO oxide powders, differentpreparation method yields different material with different atomicstructure and physical property. The material made using the method ofthe present invention shows glass-free Pb—Te—O-based crystallineparticles while another method following prior art references yieldsonly glass particles. Consequently, the obtained different particleshave different physical property, which shows different performanceduring a sintering/firing process in associated with the application ofthe conductive paste. Specifically, for the electrically conductivepaste with the above two different particles, as temperature increasesduring the sintering/firing process, the glass-free Pb—Te—O-basedcrystalline particles go through a direct physical phase transition froma solid phase to a liquid phase while the particles with glass structureparticles in the same conductive paste go through a phase transitionfrom a solid state to a glass-softening state and stay in the softeningcondition in a range of the temperature before finally transforming intoa liquid state.

The electrolytic conductive paste of the present invention contains aglass-free corrosion binder which is a Pb—Te—O-based crystallinecompound. During the process of sintering, the Pb—Te—O based crystallinecompound glass-free corrosion binder changes liquid from solid when thetemperature reaches the melting point. Before the melting point, theglass-free corrosion binder is a solid, and will not fill the pores inthe metallic powder and hinder the discharge of the organic components,thereby solving the problem that the glass frit clogs the pores whenbeing softened at the early stage; after being melted, the glass-freecorrosion binder is in a liquid state and has a low viscosity, canrapidly flows to the bottom through voids among the metallic powder, andcan effectively etch and penetrate the antireflective insulating layeron the front side of the crystalline silicon solar cell, so that a goodohmic contact is formed between the electrically conductive metallicpowder and the crystalline silicon solar cell, and the electricallyconductive metallic powder can be effectively wet, thereby promoting thecombination of the metallic powder. As a result, a high-conductivityfront electrode of a crystalline silicon solar cell is formed.Therefore, compared with the glass frit, the corrosion is faster andfuller, and the amount of the Pb—Te—O-based crystalline compoundglass-free corrosion binder is less. Further, since the low-viscosityPb—Te—O-based compound melt easily spreads, many interfaces areprovided, so that electrically conductive contact points are increased,and the tunneling effect is improved, and the resistance is reduced. Theinventors find in the study that, if the content of the glass-freecorrosion binder in the electrically conductive paste for a frontelectrode of a crystalline silicon solar cell in the embodiments isgreater than 12 parts by weight, the P-N junction region may bepenetrated, resulting in a short circuit; if the content of theglass-free corrosion binder is less than 0.5 part by weight, theantireflective layer may be not removed completely, resulting indeterioration of the performance of the crystalline silicon solar cell.Therefore, the weight ratio of the glass-free corrosion binder in theelectrically conductive paste for a front electrode of a solar cell isin a range of 0.5 wt % to 12 wt %.

II. Metallic Powder

The electrically conductive paste for a front electrode of a crystallinesilicon solar cell of the present invention contains a metallic powder.In an embodiment, the metallic powder is a metallic powder uncoated withsilver, including at least one selected from silver, gold, platinum,copper, iron, nickel, zinc, titanium, cobalt, aluminum, chromium,palladium and rhodium or an alloy thereof. In another embodiment, themetallic powder is any one or a combination of two or more selected fromcopper, iron, nickel, aluminum, zinc, titanium, cobalt, chromium and,manganese coated with silver, the thickness of the silver coating layeris in a range of 10 to 2,000 nm, and the size of the metallic powdercoated with silver is in a range of 0.1 to 5.0 μm. In anotherembodiment, the metallic powder is a mixture of a metallic powderuncoated with silver and a metallic powder coated with silver, and theweight ratio of the metallic powder uncoated with silver to the metallicpowder coated with silver is in a range of 5:95 to 95:5. Specifically,the metallic powder is used for exerting an electrically conductiveeffect in the embodiments of the present invention, and is a componentfor forming an electrode. In a preferred embodiment, the melting pointof the metallic powder is preferably in a range of 350° C. to 2,000° C.,further preferably in a range of 450° C. to 1,800° C., and morepreferably in a range of 600° C. to 1,450° C. The inventors find in thestudy that, if the melting point of the metallic powder is lower than350° C., during sintering, the metal particles will be meltedexcessively early, and hinder the discharge of the organic carrier, andflow during sintering, so that the aspect ratio of gate lines isreduced; if the melting point of the metallic powder is higher than2,000° C., the metallic powder cannot be sintered effectively during thesintering process, too many voids exist in the electrically conductivemetallic block, resulting in a high resistance of channels anddeterioration of performance.

In a further preferred embodiment, the metallic powder is at least oneselected from silver, gold, platinum, palladium and rhodium, or at leastselected from silver, gold, platinum, palladium and rhodium doped withcopper, iron, nickel, zinc, titanium, cobalt, aluminum, chromium andmanganese, or an alloy thereof, such as manganese-copper alloy,constantan alloy and nickel-chromium alloy.

In yet a further preferred embodiment, the metallic powder is any oneselected from copper, iron, nickel, aluminum, zinc, titanium, cobalt,chromium and manganese coated with silver, that is, any one metallicparticle of copper, iron, nickel, aluminum, zinc, titanium, cobalt,chromium and manganese having a layer of silver continuously coated onthe outer surface.

In an embodiment of a metallic powder having a silver coating structure,the thickness of the sliver coating layer is preferably in a range of 1to 2,000 nm, and more preferably in a range of 2 to 1,000 nm. Theinventors find in the study that, if the thickness of the Ag layer isless than 1 nm, the Ag content is excessively low, and the contactresistance or the drain current of the electrode is significantlyincreased; if the thickness of the Ag layer is greater than 10⁴ nm, theparticle diameter of the electrically conductive metallic powder isexcessively large, since Ag is noble metal, the cost of the metallicpowder is increased, thereby increasing the cost of the crystallinesilicon solar cell. Definitely, the silver layer of the metallic powderhaving a silver coating structure may be replaced by other noble metalssuch as gold and platinum. The metallic particle coated with silver maybe an alloy of metals selected from copper, iron, nickel, aluminum,zinc, titanium, cobalt, chromium and manganese, such as manganese-copperalloy, constantan alloy and nickel-chromium alloy. The metallic powdercoated with silver is formed by plating a layer of silver on a metallicpowder of copper, iron, nickel, aluminum, zinc, titanium, cobalt,chromium or manganese. In an exemplary embodiment, one or more types ofmetallic powder of copper, iron, nickel, titanium, aluminum, cobalt,chromium, zinc or manganese having a particle diameter in a range of 0.1to 5.0 μm or an alloy thereof are placed in a dilute weak acid andimmersed for 10 to 300 s to remove the oxide layer on the surface of themetal, and then silver is plated on the metallic powder to a thicknessof about 10 to 2,000 nm by using a chemical plating method. In anexemplary embodiment, the composition of the solution and the processconditions for chemical plating are: AgNO₃: 2.4 to 14.2 g/L, ammonia:0.8 g/L, formaldehyde: 1 to 3 g/L, hydrazine hydrate: 1 to 4 g/L, acomposite dispersant: 1.0 g/L, pH value: 11, bath temperature: 60° C.,stirring speed: 1,000 r/min, drying: 50° C., 30 min. In anotherexemplary embodiment, one or more types of metallic powder of copper,iron, nickel, titanium, cobalt, aluminum, chromium, zinc or manganesehaving a particle diameter in a range of 0.1 to 5.0 μm or an alloythereof are placed in a dilute weak acid and immersed for 10 to 300 s toremove the oxide layer on the surface of the metal. The metal powder iswashed with deionized water to remove residual acid. The wet powder isdried in a vacuum oven, and then the dry metallic particles free ofoxide layer are placed in a vacuum deposition device for vacuumdeposition, to obtain the metallic powder coated with silver.

In the embodiments of the metallic powder, the size of the metallicpowder particle is first required to meet requirements for printing, forexample, not clogging the printing stencil. Therefore, preferably, theparticle diameter of the metallic powder is distributed in a range of0.1 to 5.0 μm, and if the particle diameter of the metallic powder isgreater than 5 μm, problems of clogging the printing stencil anddisconnection of the electrode easily occur; if the particle diameter ofthe metallic powder is less than 0.1 μm, the viscosity of the paste isgreatly improved, resulting in failures in normal printing. In addition,the metallic powder having the preferred particle diameter can alsoreduce the area occupied by the electrode, thereby improving the lightconversion efficiency of the solar cell, and at the same time,effectively reducing the thickness of the electrode, reducing the amountof materials and reducing the production cost.

III. Organic Carrier

The electrically conductive paste for a front electrode of a crystallinesilicon solar cell of the present invention contains an organic carrier.The organic carrier includes an organic solvent, a binder, a wetting anddispersing agent, a thixotropic agent and other functional additives.The weight ratio of the organic carrier in the electrically conductivepaste for an electrode is 5 to 25. Based on the total weight of 100parts of the organic carrier, the organic solvent accounts for 50 to 95parts by weight, the binder accounts for 1 to 40 parts by weight, thewetting and dispersing agent accounts for 0.1 to 10 parts by weight, andthe thixotropic agent and other functional additives account for 1 to 20parts by weight. In an exemplary embodiment, the organic solvent may beat least one with a medium or high boiling temperature, such as alcohol(such as terpineol, butyl carbitol), alcohol ester (such as alcoholester-12), terpene and the like. The binder may be at least one selectedfrom ethyl cellulose, polymethacrylate, alkyd resin, and the like. Thewetting and dispersing agent is used to help dispersing inorganicpowders in the organic carrier, and is not particularly limited. Thethixotropic agent is used to increase the thixotropy of the paste in theprinting process, so as to ensure the resolution of electrode patternand better aspect ratio. The thixotropic agent may be an organicthixotropic agent selected from hydrogenated castor oil derivatives orpolyamide wax. The other functional agents may be added as required,such as microcrystalline wax may be added for reducing the surfacetension, DBP may be added for improving the flexibility of the paste,and PVB may be added for improving the adhesion.

IV. Preparation Method of the Electrically Conductive Paste

The present invention provides a method for preparing an electricallyconductive paste for a front electrode of a solar cell by using a simpleprocess with easy-controlled conditions and reduced production cost. Themethod includes, based on the total weight of 100 parts, weighingmaterials of the following formulation: 1) a metallic powder 70 to 95parts; 2) an organic carrier 5 to 25 parts; and 3) a glass-freecorrosion binder 0.5 to 12 parts. The glass-free corrosion binder is aPb—Te—O based crystalline compound, having a melting temperature in arange of 440° C. to 760° C. Additionally, the method includes mixing andgrinding the weighed glass-free corrosion binder, metallic powder andorganic carrier, to obtain the electrically conductive paste for a frontelectrode of a solar cell.

An exemplary preparation process of an electrically conductive paste foran electrode is shown in FIG. 7. First, a metallic powder including ametallic powder uncoated with silver and a metallic powder coated withsilver is weighed; next, a lead oxide and a tellurium oxide are weighed,and a glass-free corrosion binder is prepared, where the method forpreparing the glass-free corrosion binder is as described above; theglass-free corrosion binder and the metallic powder are premixed. Then,an organic compound is weighed, and an organic carrier is prepared.Finally, the premixed glass-free corrosion binder and metallic powder ismixed with the organic carrier, and the resulting mixture is ground toobtain the electrically conductive paste for an electrode. It should benoted that FIG. 7 only shows an example, which should not be used tolimit the scope of the present invention.

Several exemplary embodiments of preparation of an electricallyconductive paste for an electrode are described below. In an embodiment,first, a weighed glass-free corrosion binder and a weighed metallicpowder are premixed, the mixture is mixed with a weighed organiccarrier, and the resulting mixture is ground to obtain the electricallyconductive paste for a front electrode of a crystalline silicon solarcell. In another embodiment, first, a weighed glass-free corrosionbinder and a weighed organic carrier are premixed, a weighed metallicpowder is added to the mixture for further mixing, and then theresulting mixture is ground to obtain the electrically conductive pastefor a front electrode of a crystalline silicon solar cell. In anotherembodiment, first, a weighed metallic powder and a weighed organiccarrier are premixed, a glass-free corrosion binder is added to themixture, and then the resulting mixture is ground to obtain theelectrically conductive paste for a front electrode of a crystallinesilicon solar cell. In another embodiment, first, a weighed metallicpowder and part of a weighed organic carrier are premixed, a glass-freecorrosion binder and the rest weighed organic carrier are premixed, thetow premixed mixtures are mixed, and then the resulting mixture isground to obtain the electrically conductive paste for a front electrodeof a crystalline silicon solar cell.

The electrically conductive paste for a front electrode of a solar cellof the present invention contains a glass-free crystalline corrosionbinder. The amount of the crystal-based glass-free corrosion binder asthe functional additive in the conductive paste is controlled between0.5 to 12 parts by weight, such as 1 part by weight, 4 parts by weight,8 parts by weight and 10 parts by weight. Of course, there are manyvariations, alternatives, and modifications. For example, in a preferredembodiment, the glass-free corrosion binder is controlled within a rangeof 1 to 10 wt %. In another preferred embodiment, the glass-freecorrosion binder is controlled within a range of 3 to 8 wt %. Theglass-free corrosion binder of the present invention is one or acombination of two or more selected from the following Pb—Te—O basedcrystalline compounds: PbTe₄O₉, PbTeO₃.0.33H₂O, PbTeO₃, PbTeO₄, PbTe₃O₇,PbTe₅O₁₁, Pb₂TeO₄, Pb₂Te₃O₇, Pb₂Te₃O₈, Pb₃TeO₅, Pb₃TeO₆, Pb₃Te₂O₈.H₂O,Pb₄Te_(1.5)O₇, Pb₅TeO₇, Pb₆Te₅O₁₈.5H₂O, PbTe₂O₅, PbH₄TeO₆, PbTeCO₅ andPb₃TeN₂O₈, and these Pb—Te—O based crystalline compounds. Selection ofthe Pb—Te—O-based crystalline compound for the glass-free corrosionbinder is partially based on that its melting temperature usually isaround 500° C. or higher which is high enough to let substantially allorganic carrier to release without being clogged in the paste structure(hence to degrade the sintering process). When several Pb—Te—O crystalcompounds are mixed, the melting point of the mixture may be lower fromthe phase diagram analysis. In certain embodiments, the selectedPb—Te—O-based crystalline glass-free corrosion binder has a meltingtemperature in a preferred range of 440-760° C. as a functional additivein the conductive paste for manufacturing electrode on semiconductordevice.

In any conductive paste for forming electrodes of semiconductor devices,metallic powder is one major component designated as electricalconductive medium of the electrode. In an embodiment, the metallicpowder of the present invention is a metallic powder uncoated withsilver, including at least one or a combination of two or more selectedfrom silver, gold, platinum, copper, iron, nickel, zinc, titanium,cobalt, aluminum, chromium, manganese, palladium and rhodium. In anotherembodiment, the metallic powder of the present invention is one or acombination of two or more selected from copper, iron, nickel, aluminum,zinc, titanium, cobalt, chromium and manganese coated with silver, wherethe thickness of the silver coating layer is in a range of 2 to 2,000nm. The size of the metallic powder coated with silver is in a range of0.1-5.0 μm. In another embodiment, the metallic powder of the presentinvention is a mixture of a metallic powder uncoated with silver and ametallic powder coated with silver, and the weight ratio of the metallicpowder uncoated with silver to the metallic powder coated with silver isin a range of 5:95 to 95:5.

In the electrically conductive paste for a front electrode of a solarcell of the present invention, a glass-free corrosion binder having amelting temperature in a range of 440° C. to 760° C. is used. Duringuse, when the sintering temperature of the electrically conductive pastereaches the melting temperature of the glass-free corrosion binder, theglass-free corrosion binder is rapidly melted and converted from a solidstate of crystal into a liquid state, and effectively corrodes andpenetrates the antireflective insulating layer on the front side of thecrystalline silicon solar cell, so that a good ohmic contact is formedbetween the electrically conductive metallic powder and the crystallinesilicon solar cell, and the electrically conductive metallic powder iseffectively wet, thereby promoting the combination of the metallicpowder. As a result, a high-conductivity front electrode of acrystalline silicon solar cell is formed.

The electrically conductive paste for a front electrode of a crystallinesilicon solar cell of the present invention contains a glass-freecorrosion binder. During the process of sintering the electricallyconductive paste, when the temperature reaches the melting point of theglass-free corrosion binder, the glass-free corrosion binder isconverted from a solid state into a liquid state, rapidly deposits onthe surface of the antireflective layer and reacts with theantireflective layer fully in a short period of time. Through the rapidconversion of the physical form of the glass-free corrosion binder fromthe solid state into the liquid state, the clogging problem caused bythe glass frit in softening state is solved, and a space for dischargingthe organic carrier is solved. By means of this type of solidstate-liquid state combination, not only the antireflective insulatinglayer on the front side of the crystalline silicon solar cell can beeffectively corroded and penetrated, but also the organic carrier can beeasily discharged, and at the same time, the metallic powder iseffectively sintered to have a more compact structure, thereby improvingthe soldering strength and the bulk conductivity.

In order to make the technical problems to be solved, the technicalsolutions and the beneficial effects of the present invention more clearand comprehensive, the present invention is further described in detailbelow with reference to exemplary embodiments. It should be noted thatthe specific embodiment described herein are merely used for illustratethe present invention, but not intended to limit the present invention.

Embodiment 1

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a solar cell contains: 8.3 parts of aglass-free corrosion binder, 81.7 parts of a metallic powder and 10parts of an organic carrier. The glass-free corrosion binder is a PbTeO₃compound; the metallic powder is a silver powder, and the organiccarrier includes, based on the total weight of 100 parts, 70 parts of aterpineol organic solvent, 14 parts of an ethyl cellulose binder, 10parts of a wetting and dispersing agent, 5.5 parts of a thixotropicagent and 0.5 part of microcrystalline wax. Preparation of theglass-free corrosion binder includes: preparing a hot tellurous acidsolution having a concentration of 0.1 mol/L and heating to 90° C.,adding a lead acetate solution having a concentration of 0.1 mol/L inproportion (the molar ratio of tellurous acid to lead acetate being1:1), heating the mixture solution for reaction, to obtain a precipitateof the PbTeO₃ compound. The metallic powder is a silver powder, and hasa particle size in a range of 1 to 3 μm. After weighing the glass-freecorrosion binder, the metallic powder and the organic carrier accordingto the above formulation, the glass-free corrosion binder and theorganic carrier were mixed uniformly, and then the metallic powder wasadded and mixed uniformly, and finally the mixture was ground to aparticle diameter of less than 5 μm by using a three-roll mill, toobtain the electrically conductive paste for a front electrode of acrystalline silicon solar cell.

Embodiment 2

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a solar cell contains: 7.3 parts of aglass-free corrosion binder, 81.7 parts of a metallic powder and 11parts of an organic carrier. The glass-free corrosion binder is aPb₃TeO₅ compound, the metallic powder is a silver-coated nickel powder,the organic carrier includes, based on the total weight of 100 parts, 50parts of an alcohol ester-12 organic solvent, 40 parts of apolymethacrylate binder, 5 parts of a wetting and dispersing agent, 4parts of a thixotropic agent and 1 part of DBP. Preparation of theglass-free corrosion binder includes: at a proportion of a molar ratioof PbO and TeO₂ being 3:1, continuously introducing Pb₃Te alloy vaporinto a reaction chamber filled with oxygen atmosphere of 1,300° C., andinducing a chemical reaction, to obtain a powder of a Pb—Te—O-basedcrystalline compound deposited at the bottom of the chamber, andcollecting a Pb₃TeO₅ compound. The metallic powder is prepared by usinga chemical plating method, including: plating silver on a nickel powderto about 200 nm, where the particle size of the nickel powder is in arange of 0.5 to 3 μm. After weighing the glass-free corrosion binder,the metallic powder and the organic carrier according to the aboveformulation, the metallic powder and the organic carrier were mixeduniformly, and then the glass-free corrosion binder was added and mixeduniformly, and finally the mixture was ground to a particle diameter ofless than 5 μm by using a three-roll mill, to obtain the electricallyconductive paste for a front electrode of a crystalline silicon solarcell.

Embodiment 3

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a solar cell contains: 8.8 parts of aglass-free corrosion binder, 80 parts of a metallic powder and 11.2parts of an organic carrier. The glass-free corrosion binder is aPbTeCO₅ compound, the metallic powder is silver-coated copper, and theorganic carrier includes, based on the total weight of 100 parts, 65parts of a terpineol organic solvent, 20 parts of an alkyd resin binder,0.1 part of a wetting and dispersing agent, 2.5 parts of a thixotropicagent 1 and 2.4 parts of PVB. Preparation of the glass-free corrosionbinder includes: in CO₂ atmosphere, placing PbO and TeO₂ at a proportionof a molar ratio of being 1:1 in a high-temperature (1,200° C.) furnace,and reacting for 1 hour, naturally cooling the resulting product, andpulverizing and grinding, to obtain the PbTeCO₅ compound. The metallicpowder is a silver-coated copper powder, and is prepared by using achemical plating method, including: plating silver on a copper powder toabout 200 nm, where the particle size of the copper powder is in a rangeof 0.5 to 3 μm. After weighing the glass-free corrosion binder, themetallic powder and the organic carrier according to the aboveformulation, the metallic powder and the glass-free corrosion binderwere mixed uniformly, and then the organic carrier was added and mixeduniformly, and finally the mixture was ground to a particle diameter ofless than 5 μm by using a three-roll mill, to obtain the electricallyconductive paste for a front electrode of a crystalline silicon solarcell.

Embodiment 4

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a solar cell contains: 3.4 parts of aglass-free corrosion binder, 86.6 parts of a metallic powder and 10parts of an organic carrier. The glass-free corrosion binder is a PbTeO₃compound, the metallic powder is a titanium powder, and the organiccarrier includes, based on the total weight of 100 parts, 60 parts of abutyl carbitol organic solvent, 15 parts of an ethyl cellulose binder, 5parts of a wetting and dispersing agent, 15 parts of a thixotropic agentand 5 parts of PVB. Preparation of the glass-free corrosion binderincludes: placing PbO and TeO₂ at a molar ratio of 1:1 in ahigh-temperature (500° C. to 900° C.) furnace, and reacting for 1 hour,naturally cooling the resulting product, and pulverizing and grinding,to obtain the PbTeO₃ compound. The metallic powder is a titanium powderhaving a particle size in a range of 0.5 to 10 μm. After weighing theglass-free corrosion binder, the metallic powder and the organic carrieraccording to the above formulation, the metallic powder and theglass-free corrosion binder were added into the organic carrier, andmixed uniformly, and then the mixture is ground to a particle diameterof less than 5 μm by using a three-roll mill, to obtain the electricallyconductive paste for a front electrode of a crystalline silicon solarcell.

Embodiment 5

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a solar cell contains: 9.8 parts of aglass-free corrosion binder, 72 parts of a metallic powder and 18.2parts of an organic carrier. The glass-free corrosion binder is aPbTeN₂O₈ compound, the metallic powder is a cobalt powder, and theorganic carrier includes, based on the total weight of 100 parts, 70parts of an alcohol ester-12 organic solvent, 15 parts of an ethylcellulose binder, 2 parts of a wetting and dispersing agent, 8 parts ofa thixotropic agent and 5 parts of PVB. Preparation of the glass-freecorrosion binder includes: placing PbO and TeO₂ at a molar ratio of 1:1in a high-temperature reactor in NO₂ atmosphere and melting the mixtureat 950° C. for 1 hour, and then naturally cooling the resulting product,and pulverizing and grinding, to obtain the PbTeN₂O₈ compound. Themetallic powder is a cobalt powder, and has a particle size in a rangeof 0.5 to 3 μm. After weighing the glass-free corrosion binder, themetallic powder and the organic carrier according to the aboveformulation, the metallic powder and the glass-free corrosion binderwere added into the organic carrier, and mixed uniformly, and then themixture was ground to a particle diameter of less than 5 μm by using athree-roll mill, to obtain the electrically conductive paste for a frontelectrode of a crystalline silicon solar cell.

Embodiment 6

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a solar cell contains: 8.9 parts of aglass-free corrosion binder, 85.6 parts of a metallic powder and 5.5parts of an organic carrier. The glass-free corrosion binder is aPb₂Te₃O₈ compound, the metallic powder is platinum, and the organiccarrier includes, based on the total weight of 100 parts, 95 parts of analcohol ester-12 organic solvent, 1 part of an ethyl cellulose binder, 3parts of a wetting and dispersing agent and 1 part of a thixotropicagent. Preparation of the glass-free corrosion binder includes: placingPbO and TeO₂ at a molar ratio of 2:3 in a vacuum high-temperaturereactor and melting the mixture at 900° C. for 1 hour, and thennaturally cooling the resulting product, and pulverizing and grinding,to obtain the Pb₂Te₃O₈ compound. The metallic powder is prepared byplating silver on an aluminum powder to about 200 nm through a chemicalplating method, where the particle size of the aluminum powder is in arange of 0.5 to 3 μm. After weighing the glass-free corrosion binder,the metallic powder and the organic carrier according to the aboveformulation, the metallic powder and the glass-free corrosion binderwere added into the organic carrier, and mixed uniformly, and then themixture was ground to a particle diameter of less than 5 μm by using athree-roll mill, to obtain the electrically conductive paste for a frontelectrode of a crystalline silicon solar cell.

Embodiment 7

Based on the total weight of 100 parts, the electrically conductivepaste for a front electrode of a crystalline silicon solar cellcontains: 5 parts of a glass-free corrosion binder, 80 parts of ametallic powder and 15 parts of an organic carrier. The glass-freecorrosion binder is a PbTeO₃ compound, the metallic powder issilver-coated chromium, and the organic carrier includes, based on thetotal weight of 100 parts, 70 parts of an alcohol ester-12 organicsolvent, 15 parts of an ethyl cellulose binder, 2 parts of a wetting anddispersing agent, 10 parts of a thixotropic agent and 3 parts of PVB.Preparation of the glass-free corrosion binder includes: placing PbO andTeO₂ at a molar ratio of 1:1 in a vacuum high-temperature reactor andmelting the mixture at 900° C. for 1 hour, and then naturally coolingthe resulting product, and pulverizing and grinding, to obtain thePbTeO3. The metallic powder is prepared by plating silver on a chromiumpowder to about 200 nm through a chemical plating method, where theparticle size of the chromium powder is in a range of 0.5 to 3 μm. Afterweighing the glass-free corrosion binder, the metallic powder and theorganic carrier according to the above formulation, the metallic powderand the glass-free corrosion binder were added into the organic carrier,and mixed uniformly, and then the mixture was ground to a particlediameter of less than 5 μm by using a three-roll mill, to obtain theelectrically conductive paste for a front electrode of a crystallinesilicon solar cell.

The above descriptions are merely preferred embodiments of the presentinvention, and are not used to limit the present invention. Anymodifications, equivalent substitutions and improvements made withoutdeparting from the spirit and principle of the present invention shallfall within the protection scope of the present invention.

What is claimed is:
 1. A method for forming a conductive pastecomprising: providing a plurality of metal particles with a weightcomposition ranging from 70 wt % to 95 wt % based on a predeterminedtotal weight; providing an organic carrier with a weight compositionranging from 5 wt % to 25 wt % based on the predetermined total weight;providing a plurality of Pb—Te—O-based crystalline particles as asubstantially glass-free corrosion binder with a weight compositionranging from 0.5 to 12 wt % based on the predetermined total weight;mixing the plurality of metal particles, the glass-free corrosionbinder, and the organic carrier to form a mixture material; and grindingthe mixture materials to obtain a conductive paste.
 2. The method ofclaim 1 wherein the glass-free corrosion binder comprises one or acombination of two or more selected from the following Pb—Te—O basedcrystalline compounds: PbTe₄O₉, PbTeO₃.0.33H₂O, PbTeO₃, PbTeO₄, PbTe₃O₇,PbTe₅O₁₁, Pb₂TeO₄, Pb₂Te₃O₇, Pb₂Te₃O₈, Pb₃TeO₅, Pb₃TeO₆, Pb₃Te₂O₈.H₂O,Pb₄Te_(1.5)O₇, Pb₅TeO₇, Pb₅TeO₇, Pb₆Te₅O₁₈.5H₂O, PbTe₂O₅, PbH₄TeO₆,PbTeCO₅ and Pb₃TeN₂O₈, characterized by a fixed melting point, themelting point being one value within a range from 440° C. to 760° C.depending on variation of said combination.
 3. The method of claim 1wherein providing a plurality of Pb—Te—O-based crystalline particlescomprises: mixing a telluric acid solution and a lead acetate solutionto form a mixed solution, wherein the molar ratio of Te to Pb in themixed solution is in a range of 0.1:10 to 10:0.1; stirring the mixedsolution at temperature between 80° C. and 120° C. using a stirringspeed ranging from 1,000 to 1,500 r/min for 2 to 5 hours, to generate aprecipitate; collecting the precipitate as a solid material throughsolid-liquid separation; washing the solid material using filteredwater, till that a pH value of the filtrated water is in a range of 5 to7; drying the solid material at about 150° C. for 2 to 3 hours, toobtain the Pb—Te based crystalline compound; and pulverizing the Pb—Tebased crystalline compound to obtain the plurality of Pb—Te—O-basedcrystalline particles.
 4. The method of claim 1 wherein providing aplurality of Pb—Te—O-based crystalline particles alternativelycomprises: introducing PbTe alloy vapor into a reaction chamber filledwith oxygen atmosphere; reacting the PbTe alloy vapor with oxygen at atemperature ranging from 1,000° C. to 1,400° C. for 1 to 4 hours to forma reaction product; cooling the reaction product to 25° C. by naturalconvection to obtain a Pb—Te—O based crystalline compound; andpulverizing the Pb—Te based crystalline compound to obtain the pluralityof Pb—Te—O-based crystalline particles.
 5. The method of claim 1 whereinproviding a plurality of Pb—Te—O-based crystalline particlesalternatively comprises: heating a tellurium oxide and a lead oxide in anon-reducing atmosphere comprising oxygen, air, nitrogen, and argon gas,to a temperature between 700° C. and 1,000° C. to form a reactionproduct; cooling the reaction product to 25° C. by natural convection inair to obtain Pb—Te—O-based crystal compounds; pulverizing thePb—Te—O-based crystal compounds to small chunks; and grinding the smallchunks to obtain the plurality of Pb—Te—O-based crystalline particles.6. The method of claim 1 wherein providing a plurality of Pb—Te—O-basedcrystalline particles alternatively comprises: melting a tellurium oxideand a lead oxide in a vacuum atmosphere, at a temperature between 700°C. and 1,000° C. to from a product material; cooling the productmaterial by natural convection to 25° C.; and pulverizing and grindingthe product material to obtain the plurality of Pb—Te—O basedcrystalline particles.
 7. The method of claim 1 wherein the plurality ofPb—Te—O based crystalline particles has particle sizes ranging from 0.1μm to 15.0 μm.
 8. The method of claim 1 wherein the plurality of metalparticles comprises one or more metals selected from silver, gold,platinum, copper, iron, nickel, zinc, titanium, cobalt, chromium,aluminum, manganese, palladium and rhodium.
 9. The method of claim 1wherein the plurality of metal particles comprises one or more metalsselected from copper, iron, nickel, zinc, titanium, cobalt, chromium,aluminum and manganese and respectively coated with a thickness ofsilver ranging from 10 nm to 2,000 nm.
 10. The method of claim 1 whereinthe plurality of metal particles is a mixture of a first powder withoutsilver coating and a second powder with silver coating; wherein thefirst powder without silver coating comprises a first plurality ofparticles made from one or more metals selected from silver, gold,platinum, copper, iron, nickel, zinc, titanium, cobalt, chromium,manganese, palladium and rhodium, and the second powder with silvercoating comprises a second plurality of particles made from one or moremetals selected from copper, iron, nickel, zinc, titanium, cobalt,chromium, aluminum and manganese with each particle being coated with asilver layer ranging from 10 nm to 2,000 nm; wherein a weight ratio ofthe first powder without silver coating to the second powder with silvercoating is in a range of 5:95 to 95:5.
 11. The method of claim 1 whereinthe plurality of metal particles has particle sizes ranging from 0.1 μmto 5.0 μm.